It is known to extract aroma fractions from roasted coffee and to utilize them as flavoring agents for coffee extract powders.
In one process roasted coffee is freed from volatile aroma constituents by means of water vapor or wet inert gases at low elevated pressures, whereby an aroma-containing condensate is obtained which is added to the thick coffee juices produced in the usual manner before or after the spray or freeze drying. This method has the disadvantage that the readily volatile aroma constituents which are sensitive to moisture come into contact with water and thus a number of undesirable reactions which can take place in the aqueous phase are initiated, for example condensations and hydrolysis reactions. In addition, a large proportion of the aroma is lost again, particularly in the case of spray drying.
In another method an aroma oil is extracted from roasted coffee by expression with the aid of worm presses or the like and is incorporated in the finished extracts, optionally in enriched form. Despite the fact that they contain an improved proportion of aroma constituents of medium and low volatility, these extracts nevertheless have the disadvantage of having a more or less intensive burning smell which is caused by the high pressures and temperatures occurring during pressing in the interior of the pressed mass and the consequent partial pyrolysis. In addition, because of the pressing conditions the content of readily volatile fractions is lower in these extracts than in fresh roasted coffee. Although the residues from the pressing process still have a high extract content, they cannot be directly processed in the usual extraction plants, since the proportion of fine material contained therein is very high and would lead to the clogging of the filters, for which reason the pressing residues would have to be converted into extractable coarse-grained material by further pressing.
In addition, aroma constituents have been obtained from roasted coffee by solvent extraction, the polarity of the solvent being so adjusted that as many aroma fractions as possible are extracted. The disadvantage of this process consists mainly in the difficulty of separating the solvents quantitatively from the aroma constituents; this is not possible without loss, particularly of the readily volatile components.
Finally, it is known for volatile aroma substances to be extracted from vegetable material, such as roasted coffee, with gaseous carbon dioxide at pressures of about 5 to 40 atmospheres, thereupon condensing the gaseous CO.sub.2 and adding the condensate, together with the volatile aroma substances contained in it, to the coffee extract for the purpose of flavoring the latter. The volatile aroma substances can also be extracted with liquid CO.sub.2, in which case some fatty constituents are also extracted in addition to the volatile aroma constituents obtained by extraction with gaseous CO.sub.2. However, only negligibly small amounts of such fatty substances are produced by this means. As a practical matter, this process produces extracts which contain only the relatively volatile aroma producing substances as their major constituents.